Nitride semiconductors are characterized by their high saturation electron velocity and wide band gap. Thus, the nitride semiconductors have extensively been investigated, aiming at applying them to high-breakdown-voltage, high-output semiconductor devices making use of these characteristics. For example, GaN, which is nitride semiconductor, has a band gap of 3.4 eV, which is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV). Accordingly, GaN has a large breakdown field strength. GaN is therefore very promising as a material for composing compound semiconductor devices for power supply apparatus, operable under high voltage and capable of yielding large output.
Numerous reports have been made on the semiconductor devices making use of the nitride semiconductor, typified by field effect transistors, and especially by high electron mobility transistor (HEMT). Among a GaN-based HEMT, an AlGaN/GaN-based HEMT using GaN for an electron channel layer and using AlGaN for an electron supply layer attracts a lot of attention. In the AlGaN/GaN-based HEMT, lattice distortion occurs in the AlGaN layer due to difference in lattice constants between GaN and AlGaN. The distortion induces piezo polarization and spontaneous polarization of AlGaN, and thereby generates a high-density, two-dimensional electron gas (2DEG). The AlGaN/GaN-based HEMT is therefore expected as high-efficiency switching devices, and high-breakdown-voltage power devices for electric vehicles, and so forth.
However, it is difficult to obtain normally-off transistors due to high density of the two-dimensional electron gas. Investigations into various techniques have therefore been directed to solve the problem. Conventional proposals include a technique of vanishing the two-dimensional electron gas by forming an InAlN layer between the gate electrode and the electron supply layer.
The InAlN layer, however, if formed so as to extend in a region, in planar view, between the gate electrode and the source electrode, and in a region between the gate electrode and the drain electrode, may vanish the 2DEG also in these regions (access areas), and may thereby elevate the on-resistance. Conventional investigations have been directed to remove the InAlN layer in the access areas by dry etching. The dry etching of the InAlN layer in the access areas, however, induces current collapse, and thereby makes it difficult to obtain a sufficient level of drain current.    [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2009-76845    [Patent Literature 2] Japanese Laid-Open Patent Publication No. 2007-19309